This invention relates to annular seals used for providing a seal between opposed journal and cone surfaces in a rock bit or drill bit for drilling oil wells or the like and, more particularly, to a seal that is specially constructed to resist being extruded from a seal cavity in such rock bit.
Heavy-duty drill bits or rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, minerals and the like. Such drill bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. As such a rock bit is used for drilling in hard, tough formations, high pressures and temperatures are encountered.
When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the service life of a drill bit in a rock formation. Prolonging the time of drilling minimizes the time lost in xe2x80x9cround trippingxe2x80x9d the drill string for replacing the bits. Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation.
One of the consistent problems in drill bits is the inconsistency of service life. Sometimes bits are known to last for long periods, whereas bits which are apparently identical operated under similar conditions may fail within a short lifetime. One cause of erratic service life is failure of the bearings. Bearing failure can often be traced to failure of the annular seal that retains lubricant in the bearing. Lubricant may be lost if the seal fails, or abrasive particles of rock may work their way into the bearing surfaces, causing excessive wear.
Rock bit annular seals are being called on to perform service in environments which are extremely harsh. Modern bits are being run at exceptionally high surface speeds, sometimes more than 500 feet per minute, with cone speeds averaging in the range of from 200 to 400 revolutions per minute. One face of the annular seal is exposed to abrasive drilling fluid and mud. The life of the annular seal may be significantly degraded by high temperatures due to friction (as well as elevated temperature in the well bore) and abrasion.
Another factor that is known to limit the life of the annular seal within a rock bit is the differential pressure imposed on the seal in certain rock bit embodiments. Such differential pressure can cause the seal to be extruded outwardly from is placement within the rock bit. While single seal-type rock bits are typically known to include means for equalizing the pressures imposed on opposed sides of the seal to minimize and even eliminate such differential pressure, dual-seal type rock bits often do not include such pressure equalizing means for reasons of packaging constraints. A typical dual-seal rock bit includes a first or primary seal positioned adjacent the journal bearing, and a secondary seal positioned next to the first seal but adjacent the outside environment. While the primary seal serves to prevent the migration of lubricant from the journal bearing, the secondary seal serves to prevent or control the entry of drilling mud and debris into the cone and to the primary seal.
During operation of such dual-seal rock bit it is known that a relatively large pressure differential can exist between the two seals, thereby imposing an outwardly directed force onto one or both of the seals. This pressure force can cause one or both of the seals to be extruded outwardly from its respective placement in the rock bit, thereby causing the seal and ultimately the rock bit to fail.
It is, therefore, desirable that an annular seal for use in a rock bit be constructed in a manner that can minimize and/or prevent extrusion from its placement within the rock caused from differential or other pressure forces. It is desired that the annular seal be configured to provide such anti-extrusion performance without compromising its sealing performance. It is also desired that such an annular seal be configured in a manner that enables its retrofit placement in existing rock bits without the need for modification.
Annular seals of this invention are specially configured to minimize or eliminate the possibility of seal extrusion from a seal gland within a rock bit. Annular seals of this invention generally comprise an elastomeric seal body that is configured to fit within a seal gland of a rock bit. The seal body is formed from an elastomeric material an is configured having a first seal surface, for providing a seal along a dynamic rotary surface formed between the seal body and one portion of the rock bit, and a second seal surface, for providing a seal between the seal body and another portion of the rock bit.
The annular seal further comprises an extrusion prevention member that is positioned adjacent a surface of the seal body between the first and second seal surfaces. The extrusion prevention member can be integral, partially-attached, or independent of the seal body. The extrusion prevention member is preferably formed from a material having a hardness that is greater than that of the seal body. The member is positioned along the seal body at a location adjacent a groove, formed between opposed members of the rock bit, to act as a physical barrier to prevent the seal from being extruded therethrough.
Annular seals configured comprising the extrusion prevention member enjoy a lengthened service live, when compared to conventional rock bit seals, as they do not suffer the nibbling and tearing caused by being extruded into the groove during rock bit operation, which nibbling and tearing can and does reduce seal sealing area and compromise sealability.